Step outside on a hot summer day in a bustling city and it will hit you - heat radiating from the sidewalk, buildings, and streets like an inferno. However, a short drive just a few miles outside town, and it’s noticeably cooler. That difference is called the Urban Heat Island effect, and it's one of the clearest ways cities change the weather around them.

An Urban Heat Island (UHI) happens when cities and urban areas are significantly warmer than nearby rural or suburban regions. This is especially noticeable at night. City centers can run up to 10 degrees or hotter than the surrounding countryside.

This happens because cities are built from heat-absorbing materials like concrete, asphalt, steel, and brick. These surfaces soak up the sun’s energy during the day and then slowly release it at night (kind of like a heat sponge). This keeps the concrete jungles of the world hotter at night than nearby countrysides.

Natural landscapes like forests, grasslands, or farmland reflect more sunlight and cool themselves through a process called evapotranspiration, where water evaporates from soil and plants to lower the surrounding temperature.

That overnight warmth from urban heat islands isn't just uncomfortable, it can be downright dangerous during heat waves, when people rely on nighttime cooling for relief.

Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.